Introduction to the PERT Formula Series

For the math-phobic among us, any project management tool containing the word "Formula" in it can strike fear in even the most competent project manager. This series of articles aims to take this fear away and demystify the PERT Formula. Hopefully, it will even convince you of some of the benefits of becoming comfortable using it.

Articles in this series include discussions of:

When to use the PERT Formula

The benefits and disadvantages of the PERT Formula

How the PERT Formula functions with Project Management Software including Microsoft Project 2007

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What is the PERT Formula?

The PERT Formula is most commonly used in project management for estimating project durations.

One instantiation of the formula averages out three numbers: an optimistic estimate, the most likely estimate, and a pessimistic estimate.

These numbers are added together and divided by six. This calculation is performed for each action item that requires a duration estimate. Project Managers can also calculate the standard deviation from this number - the difference between the pessimistic estimate and optimistic estimate divided by six.

Once these two numbers have been determined for each task in the project (if using bottom-up estimation, which is recommended), then they can be added up to find the estimated duration for the entire project.

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What does the PERT Formula do?

The PERT Formula gives the project manager a duration you can start with when planning the project. Experienced project managers will have a more accurate idea of how long particular tasks take within a project. New project managers, however, may have a wider standard deviation when it comes to the estimated project duration due to inexperience.

The goal is not to pull numbers out of thin air, but rather, to come as close to reality as possible when determining the estimated duration for a project and for the various project phases.